PEER-REVIEWED REVIEW ARTICLE bioresources.com Bacterial Cellulose: A Sustainable Source to Develop Value-Added Products – A Review Alejandra Margarita Arévalo Gallegos,a Sonia Herrera Carrera,a Roberto Parra,a Tajalli Keshavarz,b and Hafiz M. N. Iqbal a,* In recent decades, worldwide economic and environmental issues have prompted research scientists to re-direct their interests to bio-based resources, which are sustainable in nature. In this context, microbial polysaccharides, such as bacterial cellulose (BC), also known as microbial cellulose (MC), are some of the upcoming and emergent resources and have potential application in various bio- and non-bio-based sectors of the modern world. Many researchers have already established novel BC/MC production methods, and many new studies have been published on lab- scale and large-scale production aspects of BC/MC to date. To further expand the novel use of this sustainable source, significant progress toward the development of BC/MC has appeared in recent years. Specifically, there have been many publications and/or research reports on the valorization of BC/MC in the food, paper, materials, biomedical, pharmaceutical, and cosmeceutical industries, among others. This review will address the novel application aspects of BC/MC today, with the aim of demonstrating the importance of this sustainable and novel source in the development of value-added products. Keywords: Bacterial cellulose; Microbial cellulose; Food industry; Cosmetic industry; Green technology Contact information: a: School of Engineering and Science, Tecnologico de Monterrey, Campus Monterrey, Ave. Eugenio Garza Sada 2501, Monterrey, N. L., CP 64849, Mexico; b: Applied Biotechnology Research Group, Department of Life Sciences, Faculty of Science and Technology, University of Westminster, London, UK; *Corresponding author: [email protected]; [email protected] INTRODUCTION Cellulose is a most abundant, renewable, and widely used natural material. During the last few years, the development of cellulose-based products, with multi-functional characteristics, has gained considerable attention. There has also been interest in utilizing cellulose as a reinforcement material, to impart new or improve the existing mechanical characteristics of a product. Cellulose is a major constituent of plants. It is a homogenous, linear polymer of D-glucopyranose sugar units (Kumar et al. 2009; Sánchez 2009; Bertero et al. 2012; Iqbal et al. 2013) that are connected through β linkages. The average cellulose chain has a degree of polymerization of about 9,000 to 10,000 units. Cellulose possesses excellent mechanical properties, such as tensile and elastic modulus strength of approximately 16.9 GPa and 2 GPa, respectively. Approximately 65% or more of cellulose is highly oriented and crystalline and therefore is not accessible to water or other solvents. Cellulose is protected from degradation because of its close association to a sheath of matrix polymers, which include lignin and hemicellulose (Iqbal 2015). The biotransformation from a petrochemical-based economy to a bio-based green economy necessitates a novel exploitation of natural materials that are transformable into Gallegos et al. (2016). “Bacterial cellulose,” BioResources 11(2), Pg #s to be added. 1 PEER-REVIEWED REVIEW ARTICLE bioresources.com high-value-added products for bio- and non-bio sectors of the modern world. In this context, green biotechnology could provide a noteworthy solution to this problematic issue for both bio-based green economy development and a range of value-added products of interests. The present review article focuses on the potential of utilizing microbial-based BC/MC materials, as a sustainable source that aim at the generation of green, recyclable, and sustainable products. The sustainability concept is shown in Fig. 1 (Iqbal 2015). Following a brief introduction, a part of this review mainly focuses on the potential source and various characteristics of BC/MC. In the second part of this review, various industrial and biotechnological applications of BC/MC and/or BC/MC-based materials are discussed. Fig. 1. Concept of “sustainability” (Iqbal 2015) Bacterial Cellulose – Source and Characteristics Apart from plants, cellulose is biosynthesized by certain bacteria, e.g., Rhizobium spp., Agrobacterium spp., Acetobacter spp., and Alcaligenes spp. (Vandamme et al. 1998; Iqbal 2015; Iqbal et al. 2015a). The resulting cellulose is termed either bacterial cellulose (BC) or microbial cellulose (MC). Figure 2 illustrates an overview of the BC network produced by bacteria. The Acetobacter xylinum strain is able to produce cellulose within a temperature range of 25 to 30 °C and a pH range of 4.5 to 7.5 (Son et al. 2001). Many substrates have been analyzed for their potential to work as a carbon source in the production of bacterial cellulose. These include the monosaccharide, D-glucose; the disaccharide, lactose; the polysaccharide, starch; the organic acid, gluconic acid; and the alcohol, ethylene glycol (Jonas and Farah 1998; Iqbal 2015). Bacterial cellulose is a straight chain polysaccharide, with the same chemical structure as cellulose that is derived from plants. However, bacterial cellulose has the advantage of being devoid of lignin, pectin, hemicellulose, and other biogenic products that are normally associated with plant cell wall structures (Jonas and Farah 1998; Iqbal et al. 2014). Because of its high purity and special physicochemical characteristics, bacterial cellulose has applications in a wide Gallegos et al. (2016). “Bacterial cellulose,” BioResources 11(2), Pg #s to be added. 2 PEER-REVIEWED REVIEW ARTICLE bioresources.com range of sectors, including food, bio-medical ( e.g., wound care), and tissue engineering (e.g., nanocomposites) (Svensson et al. 2005; Czaja et al. 2006; Shah et al. 2013; Silva et al. 2014; Iqbal et al. 2014; Iqbal 2015; Iqbal et al. 2015b, c). Therefore, in light of the afore- mentioned characteristics, BC/MC may be a promising candidate for the development of value-added products. Fig. 2. Scanning electron microscope image of a bacteria-generated bacterial cellulose network (Iqbal 2015) An important component to consider for both forms of cellulose, either plant-based or microbial-based, is their biocompatibility and biodegradability under natural conditions. Various forms of cellulose, including pure cellulose or chemically-modified cellulose, have been widely used in the modern era of research and development. For this reason, the development of bio-based products has been a subject of interest in material science from both ecological and environmental perspectives (Bajpai et al. 2013). Among the possible alternatives, the development of composites, utilizing cellulose as a reinforcement material, are under investigation in almost every industry. There are various methods of manufacturing bio-based products, depending on the processing techniques; e.g., surface casting, ultrasonic-assisted casting, pultrusion, extrusion, injection molding, press molding, hand lay-up, filament winding, sheet molding compounding, and enzymatic grafting (Fowler et al. 2006; Iqbal et al. 2014, 2015a). Synthetic fibres, such as glass and carbon fibers, are brittle, and they are often broken into smaller fragments (Iqbal et al. 2013); meanwhile, cellulose is flexible and will not fracture during processing over sharp curvatures. This permits high-volume fraction filling during processing of cellulose with other polymers, which results in improved mechanical properties, compared to the abrasive synthetic polymers. All of the aforementioned features enable cellulose to maintain its desired characteristics for good performance. Moreover, cellulose offers the ability for surface modification, eco-friendly processing, non-toxic nature, easy handling, and no health risks, while most synthetic polymers pose significant health risks, such as skin irritation and respiratory disease (Yang et al. 2004; Iqbal et al. 2014). Cellulose and Gallegos et al. (2016). “Bacterial cellulose,” BioResources 11(2), Pg #s to be added. 3 PEER-REVIEWED REVIEW ARTICLE bioresources.com cellulose-based materials can be used for different applications, including food, paper and packaging, tissue engineering, pharmaceutical, cosmeceutical, electronics, dentistry, and medicine (Wang and Chen 2011; Mathew et al. 2012; Ul-Islam et al. 2012). Potential Applications of BC/MC From an application standpoint, a wide spectrum of microbial strains is available for the production of BC/MC, and following careful characterization, it is possible to convert BC/MC into value-added products (Iqbal et al. 2014; Iqbal 2015). In this context, many research scientists have already focused on designing and engineering ideal BC- based products for targeted applications. The biomedical applications of BC and/or BC- based materials have already been reviewed and documented in the recent literature (Fu et al. 2013; Rajwade et al. 2015). Figure 3 illustrates various biomedical applications of BC- based materials. However, many other potential applications of BC/MC have not been comprehensively reviewed. Therefore, the current paper reviews the applications of BC/MC in the food, paper, composite, and cosmetic industries, specifically summarizing the present research on BCs for their traditional applications in current biotechnology. Fig. 3. Prospects for the various biomedical applications of BC and BC-based materials Food Industry The high level of purity, change in color, change